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Report: Huge explosion kills 40 in Syria

Also in March, the U.N. Security Council voiced "grave concern over repeated incidents of cross-border fire which caused death and injury among the Lebanese population, incursions, abductions and arms trafficking across the Lebanese-Syrian border, as well as other border violations." The declaration followed a briefing by officials on how the conflict in Syria has spilled into Lebanon.

More than 600,000 Syrians have fled to neighboring Lebanon, a country of about 4 million people, according to a U.N. estimate. But the Lebanese government puts the total at more than 1 million. Whatever the true figure, there is no dispute that the influx has destabilized the area and heightened tensions.

The attack comes as the Syrian conflict is mired in a third year of unrest, which started in March 2011 when President Bashar al-Assad cracked down on peaceful protesters.

Since then, it has evolved into a civil war that has killed more than 100,000 and transformed more than 1 million others into refugees, according to the Red Cross.